Antibodies are dimeric molecules; each monomer making up the dimer comprises one light and one heavy chain. Solutions of antibody molecules exist in two forms associated with hinge heterogeneity. Using SDS-PAGE analysis of purified Mab MAb, typically the two forms are observed as two protein bands, a major band (MW approximately 150-160 kDa) and a minor band (MW approximately 75-80 kDa). This latter form is typically observed after SDS-PAGE analysis of purified IgG4 preparations, but can be identified at much lower frequencies in all IgG isotypes, including purified, recombinant MAbs (Angal et al. 1993. Mol. Immunol. 30:105; Norderhaug et al. 1990. Eur. J. Immunol. 21:2370). The larger molecular weight isoform, referred to as Form A, contains covalent interchain disulfide bonds at positions corresponding to 239 and 242, Kabat numbering system (positions 226 and 229, EU numbering system) (Kabat, E, Wu, T T, Perry, H M, Gottesman, K S, Foeller, C: Sequences of Proteins of Immunological Interest. Bethesda, US Department of Health and Human Services, NIH, 1991). The second isoform, Form B, is thought to contain no covalent linkages between the two heavy chains and an intrachain disulfide bond between the two neighboring cysteine residues as evidenced by the 75-80 kDa seen in non-reducing SDS-PAGE electrophoresis. The two heavy chains of Form B are presumably held together by strong non-covalent (e.g., ionic) interactions associated with the CH3 domain region of the molecule. These mixtures of A and B forms are not present in solutions of MAb fragments that contain an intact hinge, but lack a CH3 domain, such as, for example, F(ab)2 fragments. Typically, genetically engineered or enzymatically digested F(ab)2 MAb preparations lack the B-form, since the molecule lacks the necessary domains for maintaining non-covalent interactions (e.g., hydrogen bonding). However, they are present in MAb preparations that do contain a CH3 domain, such as IgG4, CH2 domain deleted MAb fragments (e.g., as described in 02/060955 A2) and minibodies (see, e.g., Hu et al. 1996. Cancer Research 56:3055).
The application of protein engineering techniques to therapeutic antibody design has also produced a number of antibody formats that have been shown to have altered, and in some cases, improved pharmacodynamic, biodistribution, and activity profiles. Some altered antibody molecules have been made in which the number of cysteine residues in the hinge region is reduced to one to facilitate assembly of antibody molecules as it is only necessary to form a single disulfide bond. This also provides a specific target for attaching the hinge region either to another hinge region or to an effector or reporter molecule (U.S. Pat. No. 5,677,425). The number of cysteine residues in the antibody hinge has also been increased (U.S. Pat. No. 5,677,425). Other mutated antibodies have been constructed in which the IgG1 hinge region and the CH2 domain have been replaced with the human IgG3 hinge region (WO 97/11370). These molecules contain 11 sulfhydryl groups for substitution of multiple haptens via thiol groups.
CH2 domain deleted antibodies have a molecular mass of approximately 120 kDa and have been shown to penetrate tumors significantly better than full length IgG. Minibodies, which also have deletion of the CH2 domain, have similar characteristics. These domain deleted molecules accumulate at tumor sites more efficiently than other MAb fragments, such as F(ab)′2s, but without the unfavorable pharmacodynamic profiles seen with intact IgG antibody. CH2 domain deleted antibodies consist of a VLCL light chain and a VH1 heavy chain domain and a portion of the hinge region (e.g., the upper and middle hinge) genetically fused (either directly or through a modified peptide spacer) to a CH3 domain. As an example, the biosynthesis of recombinant CH2 domain deleted ddCC49, a domain deleted antibody that recognizes the tumor associated TAG72 antigen expressed on a variety of human carcinomas, produces the A and B isoforms in approximately 50:50 distribution in cell cultures. Cells engineered to express alternative forms of CH2 domain deleted antibodies, for example, tetravalent CH2 domain deleted antibodies, minibodies, or tetravalent minibodies also express a mixture consisting of A and B isoforms and/or monomeric half-mer molecules.
Form A and Form B are extremely difficult to separate even after MAb purification, since they are composed of identical amino acids and, therefore, have identical molecular weight and similar physical and chemical properties. They cannot be separated by standard gel filtration, affinity chromatography, or ion exchange chromatography typically used to purify antibody molecules, including recombinant MAb proteins. Current manufacturing processes discard at least 50% of the total antibody produced, having a negative impact on overall yield. Moreover, the presence of the two isoforms increases efforts required for downstream processing.
The manufacture of a homogenous preparation of binding molecules is also important where the binding molecules are intended to bind to more than one type of target molecule. Such is the case with multispecific binding molecules which comprise binding sites with variable binding specificities. For example, a single bispecific binding molecule is capable of binding to and/or coupling two distinct target molecules that are not otherwise capable of being targeted by a single agent. To retain the desired bispecificity, it is vital that bispecific dimeric polypeptides are prepared with minimal amount of Form B isoforms or monomeric half-mer molecules, as these contaminants lack the desired activity of a bispecific binding molecule. Accordingly, methods which facilitate the preferential manufacture of Form A isoforms of bispecific binding molecules would be of great benefit.